The process of acid-catalyzed cleavage of cumene hydroperoxide into phenol and acetone has been of particular industrial importance for a long time. In the preparation of phenol from an alkylaryl compound like cumene by the Hock process, cumene is oxidized to cumene hydroperoxide (CHP) in a first reaction step, known as the oxidation, and the CHP is subsequently concentrated to from 65 to 90% by weight in a vacuum distillation, known as the concentration. In a second reaction step, known as the cleavage, the CHP is cleaved into a phenolic compound and a ketone by action of an acid, usually sulfuric acid. Apart from phenol as the phenolic compound and acetone as the ketone, the cleavage product further comprises additional compounds which can have been formed in the reaction steps preceding the cleavage and which are only partly transformed, if at all, in the cleavage. The most important compounds that may be present in the cleavage product in addition to phenol and acetone are, in particular, α-methylstyrene (AMS), cumene and acetophenone. In addition, small amounts of dimethyl phenyl carbinol (DMPC) formed in the oxidation can be present in the cleavage product. Further impurities include compounds such as methylbenzofuran (MBF), hydroxy acetone, mesityl oxide (MO) and carbonyl compounds such as acetaldehyde and 2-phenylpropion-aldehyde. After neutralization of the cleavage product and possible removal of an aqueous phase, the cleavage product is worked up by distillation.
Various processes for working up the cleavage product mixtures by distillation are known (Ullmann's Encyclopedia of Industrial Chemistry, 5th completely revised edition, Vol. A19, 1991, VCH Verlagsgesellschaft mbH, Weinheim). In principle, all these processes involve initial neutralization of the cleavage product mixtures using aqueous sodium hydroxide, amines or aqueous phenoxide solution. After phase separation, the organic part of the neutralized cleavage product mixture is transferred to a first column in which crude acetone is distilled off from the remaining cleavage product via the top of the column. This crude acetone is usually treated with alkali in a scrubber and once again purified by distillation. However, the scrub is sometimes also carried out in the column. The bottom product from the first column is distilled in a second column from which AMS and cumene are taken off at the top and are usually passed to a hydrogenation in which cumene is produced again. AMS and cumene can also be separated off by azeotropic distillation in the presence of water. The bottom product remaining in the second column is distilled in a crude phenol column.
The crude phenol obtained can be purified further by extractive distillation using water or by treatment with an acid ion exchanger and subsequent distillation. In the latter process, compounds that are difficult to separate from phenol by distillation, e.g., mesityl oxide and hydroxy acetone, are condensed to form higher-boiling compounds.
Such a process is described, for example, in U.S. Pat. No. 5,064,507 (assigned to Allied). In this process, the cleavage product is firstly separated from the crude acetone in a crude acetone column. The bottom product is transferred to a cumene column in which cumene and AMS are separated from the cleavage product. The column is, however, operated so that a certain proportion of AMS remains in the bottom product since this is required as reactant or solvent in the further processing of the phenol to remove MBF and other impurities. This bottom product is reacted with an amine, preferably hexamethylenediamine, in a reactor having plug flow characteristics in order to convert carbonyl compounds, e.g., acetol (hydroxy acetone) or MO, into higher-boiling compounds. The product, which has been treated in this way, is worked up further by distillation. However, it needs to pass through further four columns and two reaction zones before the purified end product phenol is obtained. The initial outlay for apparatus required for the removal of the hydroxy acetone from the phenol is relatively high in this process.
According to DE-AS 1 105 878 (Phenolchemie), the neutralized cleavage product which is separated off from the crude acetone in a crude acetone column is transferred into a hydrocarbon column in which, in the presence of water, hydrocarbons boiling lower than phenol, such as AMS and cumene are distilled off overhead. From the bottom of the column the organic phase is taken off and applied to the top of a downstream column in which water is separated off from phenol and high-boilers which are produced in the bottom of the column. The mixture of phenol and high-boilers is then transferred to a crude phenol column. The residues produced in the crude phenol column and in the pure phenol distillation can then be fed to a cracking still in which the residues are worked up and one part of phenol is recovered. These recovered products of value can be transferred again to the hydrocarbon column.
EP 0 032 255 (UOP) describes a process for working up cleavage product in which the organic part of a virtually neutralized (pH≈6) cleavage product is again washed with water and then the organic part is transferred to a crude acetone column in which the crude acetone is separated off from the remaining cleavage product. The residue remaining in the bottom phase is transferred directly to a cumene column in which the crude phenol is produced as bottoms product which in turn is fed to a purifying distillation. The mixture taken off overhead from the cumene column, which contains principally AMS, cumene and water, is transferred to a phase-separation vessel in which an aqueous phase is separated off. The resultant organic phase is transferred to a washing column in which the organic mixture is treated with sodium hydroxide solution in order to remove any phenol still present from the mixture of AMS and cumene as sodium phenoxide. The mixture of AMS and cumene which has been freed from phenol is fed via the top of the column to a hydrogenation.
In U.S. Pat. No. 4,262,150 (UOP), also, the same column circuit is used as described in EP 0 032 255 (UOP). The difference from EP 0 032 255 is that, to neutralize the cleavage product, an extraction column is used instead of one or more combinations of mixers and phase-separation apparatuses.
U.S. Pat. No. 3,322,651 (assigned to UOP), describes the use of nitrogen compounds, in particular amines, for purifying phenol obtained in the cleavage of CHP. However, the amines added likewise have to be removed again from the products and are quite expensive.
U.S. Pat. No. 5,510,543 (GE) describes a process for working up the cleavage product from CHP cleavage, in which the cleavage product is adjusted to a pH of 4.0 to 4.9 in a neutralizer by adding a base, in particular sodium hydroxide solution. In the neutralizer the cleavage product is separated into an aqueous phase and an organic phase. The organic phase is transferred to a column termed the splitter in which the cleavage product is separated by distillation into an acetone-rich fraction and a phenol-rich fraction. The phenol-rich fraction is taken off at the bottom of the column and fed to a phenol purification, which can consist, for example, of one or more further distillations. The acetone-rich fraction is taken off at the top of the column and fed to an acetone column, in which case base is added to this fraction before entry into the column in such an amount that the fraction has a pH of approximately 9, so that the organic acids which are also present in this fraction are neutralized. The mixture arising in the bottom phase of the column which, in addition to water, also contains hydrocarbons and the salts of organic acids, is transferred to a phase-separation apparatus in which this mixture is separated into an organic phase and an aqueous phase. The organic phase can be treated again for recovery of cumene.
Furthermore, processes have been developed in which individual fractions which arise in the work-up by distillation of cleavage product are treated specifically. Thus, U.S. Pat. No. 5,487,816 (UOP) describes a process for separating off AMS from a mixture which contains phenol, AMS and water and which arises as bottom product of a crude acetone column. The crude acetone column is operated in this case in such a manner that cumene is taken off from the cleavage product at the top of the crude acetone column together with the acetone. The mixture containing AMS and phenol is separated in a column in such a manner that in the bottom of the column predominantly phenol is produced, which can be fed to further work-up, and at the top of the column a mixture of AMS, water and smaller amounts of phenol is taken off, which mixture is condensed and adjusted to a pH above 6 by adding a basic reagent. This achieves the phenol being principally present in the aqueous phase, while the AMS is present in an organic phase in which only small amounts of phenol are present as impurity. The phases are separated from one another by a phase-separation apparatus. The organic phase can be fed to a hydrogenation, while the aqueous phase can be fed back to the column as reflux.
In U.S. Pat. No. 4,370,205 (UOP), the stream taken off from the bottom of the crude acetone column also still contains cumene, in contrast to the process described in U.S. Pat. No. 5,487,816 (UOP). Against this background a different column circuit is proposed. In particular two columns operated under virtually the same conditions are used in which the bottoms product produced is predominantly phenol, whereas predominantly AMS and cumene are taken off overhead. The crude phenol taken off as bottoms product in the first column is fed to further work-up steps, and the crude phenol taken off as bottoms product in the second column is fed back to the first column. The overhead product of the second column is treated with sodium hydroxide solution in a wash column. The overhead product of this column, which comprises AMS and cumene, can be fed to the hydrogenation.
GB 1 021 759 (Mitsui) describes the work-up of a cleavage product mixture, which has been obtained in the acid-catalyzed cleavage of CHP and from which the catalyst has been removed by neutralization and scrubbing. To be able to obtain a phenol having a low hydroxy acetone content, the separation is carried out by feeding the cleavage product mixture into a distillation column at a side inlet and fractionating this mixture in one separation step to give a top fraction comprising water, acetone, hydroxy acetone and cumene and a bottom fraction comprising phenol having a hydroxy acetone content of less than 100 ppm. Depending on the composition of the cleavage product mixture, cumene was added in an amount so that the ratio of cumene to phenol in the feed to the column was at least 0.28 part by weight of cumene to 1 part by weight of phenol, since the process is based on the separation of an azeotropic mixture of hydroxy acetone and cumene from the phenol. The hydroxy acetone can be removed from the phenol by this process. However, a top fraction that comprises not only water, cumene and hydroxy acetone but also acetone is obtained. Phenol contamination may also be present in the top fraction. These products likewise have to be separated from one another.
In U.S. Pat. No. 4,251,325 (assigned to BP Chemicals), the work-up of a fraction which has been substantially freed of low boilers, water, and acetone has been optimized by operating the cumene column in such a way that a mixture comprising cumene, AMS and hydroxy acetone is taken off at the top, with this mixture being separated virtually completely from the crude phenol remaining in the bottoms and thus not having to be removed in a costly fashion during the work-up of the phenol. This process gives phenol containing less than 30 ppm of hydroxy acetone. A disadvantage of this process is the fact that the input mixture has to be substantially freed of water, which is why an acetone fraction comprising low boilers and also the major part of the water present in the cleavage product mixture has to be separated from the cleavage product mixture in the preceding separation step. The work-up of such an acetone fraction by methods of the prior art is relatively uneconomical, since a high expenditure for apparatus is required.
U.S. Pat. No. 4,333,801 (UOP) describes the work-up of a fraction which comprises AMS, cumene, phenol, water and impurities, for example hydroxyacetone. This process is chiefly concerned with removing an AMS/cumene fraction, which has a very low phenol concentration, from the total fraction. This is achieved by operating the cumene column in such a manner that a mixture of cumene and AMS is taken off overhead from the column, which mixture is condensed and run into a phase-separation vessel. Any water possibly present is separated off and discarded. A portion of the organic phase is fed back to the column top as reflux. Another portion of the organic phase is fed to a washer in which phenol residues which would interfere with the hydrogenation are removed from the phase so that this phase can be fed to the hydrogenation. From a side stream takeoff of the cumene column there is taken off a fraction comprising AMS and cumene and an azeotropic mixture of water and phenol, which fraction is also condensed and transferred to a phase-separation vessel. The aqueous phase which can comprise phenol is fed to a work-up stage. The organic phase which comprises cumene, AMS and as much phenol as remains in the organic phase in accordance with the phase equilibrium between the organic and aqueous phases is evaporated and fed back in the vaporous state into the cumene column above the side stream takeoff. A crude phenol fraction is taken off from the bottom of the column.
A process for phenol rectification with energy recovery has been described in U.S. Pat. No. 5,131,984, however, the crude phenol feed has to be freed from substantial portions of acetone, cumene and AMS by conventional process, thus limiting the overall energy recovery possibilities.
A similar process is described in U.S. Pat. No. 5,122,234 (Allied) with the difference that water is additionally run into the column and at the top of the column a mixture which predominantly comprises water and phenol is taken off, which mixture is partly condensed and returned to the column.
U.S. Pat. No. 4,340,447 describe a process and apparatus for obtaining pure acetone from CHP cleavage reaction product.
From DE 100 60 505 A1 and WO 02/46133 A1, and DE 100 60 503 A1 and WO 02/46132 A1, the disclosure of which is hereby incorporated by reference, a process is known for separating phenol from a mixture comprising at least cumene, phenol and hydroxy acetone, in which process a phenol having a hydroxy acetone content of less than 30 ppm is separated from the mixture with comparatively low energy consumption, with a low outlay in terms of apparatus and in the presence of water. It is stated there that a mixture comprising at least phenol, water, hydroxy acetone and cumene can be worked up by means of a single fractional distillation step with subsequent phase separation to give phenol having a hydroxy acetone content of less than 30 ppm, with this fractional distillation step being able to be carried out with a lower energy consumption and a lower outlay in terms of apparatus than in conventional processes. Likewise, it is described that reversal of the order of the process steps also enables phenol having a hydroxy acetone content of less than 30 ppm to be separated from the abovementioned mixture.